The MongoDB Ruby driver is the officially supported Ruby driver for
MongoDB. It’s written in pure Ruby and is optimized for simplicity.
It can be used on its own, but it also serves as the basis of several
object mapping libraries, such as Mongoid.

Because MongoDB is so easy to use, the basic Ruby driver can be the best
solution for many applications.

But if you need validations, associations, and other high-level data
modeling functions then an Object Document Mapper may be needed.

In the context of a Rails application these provide functionality
equivalent to, but distinct from, ActiveRecord. Because MongoDB is a
document-based database, these mappers are called Object Document
Mappers (ODM) as opposed to Object Relational Mappers (ORM).